Print this pageAAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Deposition of Retrogradational 
Carbonate
Grainstone Shoals and Subsequent Basal Barrier (Sole Seal) Development by Deposition and Diagenesis - Lower Cretaceous Ratawi Oolite Reservoir (Minagish Formation) Wafra Field, PNZ-Kuwait
(1) Kuwait Gulf Oil Company, Ahmadi - Kuwait, Kuwait.
The Wafra field is located in the Partitioned Neutral Zone (PNZ) between Kuwait and Saudi Arabia and has been producing medium oil from the Ratawi Oolite Limestone reservoir since 1956.
The Wafra Ratawi is an example of a detached rimmed shelf carbonate shoal depositional system deposited on a paleo-high in the Late Cretaceous. The depositional architecture was analyzed for major depositional controls and to explain the existence of a sole seal (Basal Barrier) that restricted early water encroachment and contributed greatly to the productivity of the reservoir.
The carbonate platform is made up of allochem Oolitic limestone shoals (grainstones) during relative sea level stagnation in an overall transgressive retrogradation shoal system with shallowing upward sequences. The reservoir is structurally trapped on three sides and stratigraphically trapped on one side where tighter, off-shoal deeper marine carbonate wackestones and mudstones provide the necessary reservoir seal.
As the leeward and windward carbonate grainstone shoals developed, a tighter carbonate mudstone was deposited by reduced circulation of nutrients and aeration, and increased excessive heat, resulting in a low stand system tract (LST) lagoonal facies. The retrogradation of these grainstone shoals with intervening lagoonal facies resulted in a continuous development of a tighter facies across the field, enhancing a substantially facies-driven sole-seal to the reservoir called the Basal Barrier. Diagenetic cementation of the Basal Barrier (possibly due to Neomorphism or re-crystallization of pre-existing carbonate fines) further reduced the sole seal permeability restricting bottom aquifer support, resulting in lower water production for much of the production life.
As transgression continued, the grainstone forming carbonate
factory was over-whelmed and drowned by a rapid rise in sea level during the transegressive system tract (TST), and tighter, deeper marine carbonates capped the main reservoir grainstone shoals. Complete entrapment came with continued transgression during the highstand system tract (HST) and deposition of tight deeper marine argillaceous carbonates grading to calcareous shales at the top forming the cap rock to the reservoir.